The present invention relates to a process for producting (-)-dihydrochrysanthemolactone from 2-caren-4-one. The 2-caren-4-one is treated in succession with a methylating agent, an oxidizing agent, a methylating agent again, and finally a lactonizing agent. This invention also relates to the reaction intermediate compounds resulting from such treatment of 2-caren-4-one. The lactone so produced has been found to be a precursor for (+)-trans-chrysanthemic acid, which in itself is useful in the preparation of many synthetic pyrethroid insecticides.
The previously proposed route for synthesizing dihydrochrysanthemolactone is described in U.S. Pat. No. 3,565,915. However, the starting material differs in that the reference proposes beginning the synthesis with optically active 2,2-dimethyl-3-cis-(2'-oxopropyl) cyclopropyl-1-acetaldehyde. Additionally, during the synthesis, the compound is converted into optically active cis-homocaronic acid and then to optically active cis-homocaronic acid anhydride. However, a Grignard reagent is then used to convert the cis-homocaronic anhydride to the desired optically active dihydrochrysanthemolactone.
One advantage of the present invention is that it begins the synthesis of the desired product with 2-caren-4-one. This product is readily obtainable from (+)-3-carene which is available in commercial quantities. Additionally, only the desired optically active (-)-dihydrochrysanthemolactone is produced.